Diazonium compounds are well known and widely used in the preparation of negative-acting lithographic printing plates to impart light sensitivity thereto. Upon exposure, the photochemical decomposition of the diazonium salt produces physical and chemical changes, such as crosslinking, insolubility, increased adhesion, etc. Most commonly utilized diazonium compounds are the diazo resins, such as those described in U.S. Pat. No. 2,714,066, which are synthesized by the condensation reaction of an active carbonyl compound with a diazo compound.
These diazonium resins are disclosed in many patents and have been coated either alone, such as in U.S. Pat. No. 2,714,066; overcoated with a resinous layer, such as is disclosed in U.S. Pat. No. 3,136,637; overcoated with a photopolymer, such as is disclosed in U.S. Pat. No. 3,905,815; or included in conjunction with other resinous materials such as is disclosed in U.S. Pat. No. 3,660,097.
Variations in the properties of diazo resins have been achieved by employing different anions therewith, e.g., as is disclosed in U.S. Pat. No. 3,790,556, wherein the solubility properties thereof are changed as well as humidity resistance, i.e., storage life. Furthermore, simple monomolecular diazonium salts have been reacted with high molecular weight sulfonated phenol-formaldehyde resins, as disclosed in U.S. Pat. No. 3,199,981, to prepare, in association with other resins, positive printing plates from positive originals.
One common characteristic of light-sensitive lithographic plates utilizing diazo resins is that the resin has a tendency to remain or adhere in the non-image areas after chemical development or processing. This small amount of unrecovered resin results in an ink toning and scumming condition in the background of the plate during the subsequent printing process. The reasons for the unremoved diazo resin in the non-image areas are believed to be due to physical attachment thereof to the substrate, insoluble fractions, minor decomposition products, or perhaps for a combination of these reasons. This specific problem is typically solved by utilizing special chemicals in the developer, typically called desensitizing agents, which aid in the removal of this unexposed diazo resin material. For example, U.S. Pat. Nos. 3,905,815; 3,891,438; 3,891,439; and 3,669,660 disclose solutions to the foregoing problem.
Surprisingly, I have now found a light-sensitive adduct derived from a combination of a diazo resin having a plurality of pendant diazonium sites with a polymer having a plurality of sulfonated groups, the adduct being capable of eliminating the problem discussed above. While it might be anticipated that such a combination of polyionic species would result in a completely insoluble ionically crosslinked mass, not suitable for printing plate utility, such has not been the case. A soluble and highly useful material for presensitized printing plates has resulted from the combination. In fact, the use of this adduct in a presensitized printing plate construction allows for the substantial elimination of the necessity for desensitizing salts in a developer solution.
A wide variety of simple developers and techniques can be utilized to prepare a printing plate made with the adduct, such as water or water/alcohol, simple machine processors, and, in some instances, on-press processing. Obviously, the simplicity of the developer used in providing the printing plate is environmentally desirable because of the reduced necessity for desensitizing agents and conventional harsh or polluting developers.